Filter arrangements and, in turn, stop filters are of great significance in many areas of electrical engineering and particularly also in communication engineering and mobile telecommunications. Trap circuits of this type can be realised, as is known, for example by a parallel circuit of a coil and a capacitor in the form of an oscillating circuit. The filter arrangements concerned can consist, for example of a high-pass (HP), a low-pass (LP) or a band-pass (BP) which are constructed, for example from series and/or parallel circuits of L/C components.
Such filter arrangements or trap circuit filters are often used in mobile telecommunications for operating multi-band antennae in order to achieve, for example a decoupling of approximately 50 dB between the frequency bands. Thus, filters of this type can also be used, for example for intersystem decoupling in multi-band antennae, because additional stop filters are required here to achieve the aforementioned 50 dB decoupling between the frequency bands. Furthermore, a good adaptation (VSWR) and a low attenuation have to be ensured in the transmission band of the frequency band to be transmitted.
Finally, solutions are also known in which a transformation line and associated stub lines are configured as microstrip lines on a printed circuit board. Solutions of this type can be inferred as being known, for example from the prior publication “Microstrip Filters for RF/Microwave Applications”, Wiley Series in Microwave and Optical Engineering, by John Wiley & Sons, Inc., 2001, Jia-Sheng Hong and M. J. Lancaster, Chapter 6, pages 161-190 and from Chapter 5 “Lowpass and Bandpass Filters” from the same prior publication, namely pages 109 to 121.
A corresponding HF filter arrangement is also to be taken as known from U.S. Pat. No. 6,278,341 B1, for example. The filter is constructed in such a way that one or more stub lines lead out from an HF inner conductor. The inner conductor is arranged at a distance from the outer conductor. The stub lines leading out from the inner conductor are arranged directly adjacent to an outer conductor portion. In other words, the stub line is arranged on one side of a substrate, the substrate being positioned on a corresponding outer conductor surface in such a way that the stub line cooperates directly with the outer conductor.
In this respect, it is also known to construct trap circuit filters, instead of the above-mentioned microstrip stub lines which cooperate directly with an individual outer conductor, using coaxial cables. In this case, one or more stub lines are branched off from a signalling line transmitting an HF signal. For this purpose, arranged on the HF signalling line are, for example triple solder connectors, one of these soldered joints serving as the branching point for the mentioned stub line which terminates open, i.e. in open circuit. In this respect, a plurality of such stub lines can be arranged offset in the longitudinal direction of the HF signalling line and they also run towards one another, for example between two triple solder connectors and terminate freely in each case. It is then also possible to provide transformation paths.
Filters of this type using coaxial cables (also for the stub lines) are very tolerance-sensitive and cannot be optimally adjusted due to their method of construction (using the separate cable impedances and the solder connectors).